1. Field of the Invention
The invention relates to the gas phase catalytic dehydrogenation of ethane to ethylene in the presence of oxygen, i.e., the oxydehydrogenation of ethane to ethylene.
2. DESCRIPTION OF THE PRIOR ART
Ethylene has been conventionally prepared, commercially, by thermally cracking ethane in an endothermic reaction which is carried out at temperatures of about 600.degree. to 1000.degree. C. (U.S. Pat. No. 3,541,179). The reaction time in such process is very short, which makes it difficult or impossible to efficiently recover heat from the process stream. Further, the high temperatures which are used require the use of special alloys in the construction of the furnaces or the reaction vessels in which the reaction is conducted. The cracking reaction also causes the formation of relatively large amounts of low boiling by-products such as hydrogen and methane which complicates, and makes more expensive, the recovery of the ethylene from such by-products.
It is possible to oxydehydrogenate ethane by using a variety of oxyhalogenation catalyst systems in an exothermic reaction. These reactions, however, have only been accomplished at temperatures of at least about 500.degree. to 600.degree. C. (U.S. Pat. No. 3,080,435). Furthermore, the presence of the halogen atoms increases the difficulty of recovering any olefins which are produced. Also, exotic and expensive materials of construction are required to withstand corrosion by the halogens and hydrogen halides in the reaction systems. Further, the halogens themselves must be recovered and recycled to make the system economical.
The oxydehydrogenation of selected .gtoreq.C.sub.3 alkanes, at relatively high temperatures in an exothermic reaction has also been accomplished with selected catalysts which contain vanadium (U.S. Pat. Nos. 3,218,368, 3,541,179 and 3,856,881) and vanadium and molybdenum (U.S. Pat. No. 3,320,331).
The use of molybdenum and vanadium containing catalyst systems for the gas phase oxidation of alpha-beta unsaturated aliphatic aldehydes, such as acrolein, to the corresponding alpha, beta unsaturated carboxylic acids, such as acrylic acid, has been known. These catalyst systems include those containing the elements Mo, V and X, where X is Nb, Ti or Ta as disclosed in Belgian Pat. Nos. 831,322; 821,324 and 821,325.
Prior to the present invention, however, it has not been possible to readily oxydehydrogenate ethane to ethylene at relatively low temperatures with relatively high levels of conversion, selectivity and productivity.
The terms percent conversion, percent selectivity and productivity which are employed herein with respect to the present invention are defined as follows: ##EQU1##